Fix qpy for `MCX` gates (#9391)

* Add MCXGrayCode

* Add test and reno

* Add other mcx gates

* Fix test

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/qpy/binary_io/circuits.py

@@ -252,7 +252,14 @@ def _read_instruction(file_obj, circuit, registers, custom_operations, version,
     if gate_name in {"IfElseOp", "WhileLoopOp"}:
         gate = gate_class(condition_tuple, *params)
     elif version >= 5 and issubclass(gate_class, ControlledGate):
-        if gate_name in {"MCPhaseGate", "MCU1Gate"}:
+        if gate_name in {
+            "MCPhaseGate",
+            "MCU1Gate",
+            "MCXGrayCode",
+            "MCXGate",
+            "MCXRecursive",
+            "MCXVChain",
+        }:
             gate = gate_class(*params, instruction.num_ctrl_qubits)
         else:
             gate = gate_class(*params)

releasenotes/notes/fix-qpy-mcxgray-421cf8f673f24238.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed a bug in QPY (:mod:`qiskit.qpy`) where circuits containing gates of class
+    :class:`.MCXGate`, :class:`.MCXGrayCode`, and :class:`MCXRecursive`, and
+    :class:`.MCXVChain` would fail to serialize.
+    See `#9390 <https://github.com/Qiskit/qiskit-terra/issues/9390>`__.

test/python/circuit/test_circuit_load_from_qpy.py

@@ -24,7 +24,18 @@ from qiskit.circuit.classicalregister import Clbit
 from qiskit.circuit.quantumregister import Qubit
 from qiskit.circuit.random import random_circuit
 from qiskit.circuit.gate import Gate
-from qiskit.circuit.library import XGate, QFT, QAOAAnsatz, PauliEvolutionGate, DCXGate, MCU1Gate
+from qiskit.circuit.library import (
+    XGate,
+    QFT,
+    QAOAAnsatz,
+    PauliEvolutionGate,
+    DCXGate,
+    MCU1Gate,
+    MCXGate,
+    MCXGrayCode,
+    MCXRecursive,
+    MCXVChain,
+)
 from qiskit.circuit.instruction import Instruction
 from qiskit.circuit.parameter import Parameter
 from qiskit.circuit.parametervector import ParameterVector
@@ -1043,9 +1054,17 @@ class TestLoadFromQPY(QiskitTestCase):
 
     def test_standard_control_gates(self):
         """Test standard library controlled gates."""
-        qc = QuantumCircuit(3)
+        qc = QuantumCircuit(6)
         mcu1_gate = MCU1Gate(np.pi, 2)
+        mcx_gate = MCXGate(5)
+        mcx_gray_gate = MCXGrayCode(5)
+        mcx_recursive_gate = MCXRecursive(4)
+        mcx_vchain_gate = MCXVChain(3)
         qc.append(mcu1_gate, [0, 2, 1])
+        qc.append(mcx_gate, list(range(0, 6)))
+        qc.append(mcx_gray_gate, list(range(0, 6)))
+        qc.append(mcx_recursive_gate, list(range(0, 5)))
+        qc.append(mcx_vchain_gate, list(range(0, 5)))
         qc.mcp(np.pi, [0, 2], 1)
         qc.mct([0, 2], 1)
         qc.mcx([0, 2], 1)